1. Field of the Invention
The present invention relates to a zirconium oxide and lithium oxide-containing borosilicate glass of increased resistance to attack by various chemical compounds and to the use of this improved borosilicate glass in several different applications.
2. Prior Art
Glasses, which have a very high resistance to chemical attack by both acidic and alkaline media, are required for glass-metal seals, which are exposed to a chemically corrosive environment, e.g. in chemical plants or reactor engineering. The thermal expansion properties of these sealing glasses must match those of high resistance metals or alloys that are employed. It is desirable that their linear expansion coefficients be close to or slightly under that of the metals to be sealed so that compressive strains build up in the glass on cooling of the seal. These compressive strains guarantee, on the one hand, a hermetic seal and, on the other hand, prevent the build up of tensile stresses in the glass, which would otherwise cause stress-corrosion. Glasses with thermal expansion coefficients .alpha..sub.20/300 between 5.2 and 5.7.times.10.sup.-6 /K are required as sealing glasses for glass-metal seals with Fe--Ni--Co alloys, e.g. Vacon.RTM. 11, which has a thermal expansion coefficient .alpha..sub.20/300 =5.4.times.10.sup.-6 /K, zirconium (.alpha..sub.20/300 =5.9.times.10.sup.-6 /K) or zirconium alloys.
The processing temperature V.sub.A at which the viscosity of the glass is 10.sup.4 dPas is an essential parameter for characterizing the workability of a glass. It should be reduced since slight V.sub.A reductions have already led to a significant reduction in manufacturing costs, because the melting temperature is lowered. Furthermore a processing temperature V.sub.A which is as low as possible is of advantage in making glass-metal seals, since overheating of the parts to be sealed can be avoided, because they are already sealed either at a lower temperature or in a shorter time. Use of glasses with reduced or lowered processing temperatures V.sub.A can avoid volatilization and retrograde condensation of glass ingredients which can cause impairment of the sealing process and, in the worst case, unsealing. Furthermore the processing interval, i.e. the temperature difference of the processing temperature V.sub.A and the softening temperature E.sub.w, i.e. the temperature at which the viscosity of the glass amounts to 10.sup.7.6 dPas, is important. The temperature range in which the glass can be processed is also designated as the "length" or "size" of the glass.
Glasses have already been described in the chemical literature which have a high resistance to chemical attack. However these glasses do not have the desired thermal expansion coefficients with simultaneously lower processing temperatures accompanying the high resistance to chemical attack.
German Patent DE 42 30 607 C1 discloses borosilicate glasses which have a high resistance to chemical attack and which can be sealed with tungsten. They have a thermal expansion coefficient .alpha..sub.20/300 of at most=4.5.times.10.sup.-6 /K.
The borosilicate glasses described in the published patent application DE 37 22 130 A1 and the zirconium-containing borosilicate glasses described in the German Patent DD 301 821 A7 have thermal expansion coefficients of at most 5.0 or 5.2.times.10.sup.-6 /K which is too low for sealing with the above-mentioned high resistance metal.
The glasses of German Patent DE 44 30 710 C1 have a high proportion of SiO.sub.2, namely &gt;75% by weight and &gt;83% by weight SiO.sub.2 and B.sub.2 O.sub.3 in combination with a weight ratio of SiO.sub.2 /B.sub.2 O.sub.3 &gt;8, and little Al.sub.2 O.sub.3, which provides a high resistance to chemical attack but leads to a processing temperature which is disadvantageously too high.
The glasses of German Patent DE 195 35 708 C1 are highly resistant to chemical attack because of their high SiO.sub.2 content, but have a disadvantageously high processing temperature and low thermal expansion. Also its resistance to attack by alkaline media drops with a decreasing SiO.sub.2 /B.sub.2 O.sub.3 ratio, also with decreasing SiO.sub.2 content.